Electromagnetic resonators are often used in filters in order to pass or reject certain signal frequencies. To optimize filter performance, the resonators should have a minimum of signal loss in the passed frequency range. Such losses in resonators can occur in a variety of modes, but all manifest themselves through the generation of heat caused by resistance to current flowing on the surfaces of conductive elements in the resonator. For that reason, conductors in resonators are usually chosen for their low-surface resistance. However, even with low-surface resistance metals, such as copper or silver, significant heating and signal losses may occur. The heating can further increase the surface resistance of the metal, thereby adding to signal loss.
In order to minimize losses in resonators, superconducting materials have been used. For instance, if a cavity resonator is used, the walls of the cavity or a resonant element located inside the cavity may be made from or coated with a superconducting material. While superconductors have a significantly lower surface resistance than ordinary conductors, a relatively small amount of heat will still be generated in a superconducting resonator. Dissipation of that heat may not be a significant problem if the power of the filtered signal is relatively low. Thus, when a superconducting resonator is used, for instance, as a component in systems receiving low-power radio frequency signals, heat build-up in the superconductor may not have significant adverse effects. However, if the superconducting resonator is used, for instance, as a component in a high-power signal transmission system, heat build-up in the superconducting material can result in serious performance degradation.
As heat builds up in a superconducting material, the temperature of that material may rise above its critical temperature. Once a superconductor rises above its critical temperature, it loses its superconducting properties, thereby increasing the surface resistance drastically, and further generating heat until the component completely fails. This phenomenon is known as thermal runaway. Therefore, removing heat from a resonator handling relatively high power signals, particularly when superconducting materials are used, may be required for effective resonator performance. Moreover, removal of heat must be accomplished without significantly increasing the overall loss of the resonator.